1. Field of the Invention
The present invention relates to an automatic transmission mountable on a vehicle, and particularly to a structure of a speed change mechanism thereof, and belongs to a technical field of vehicle transmissions.
2. Description of the Related Art
An automatic transmission mountable on a vehicle, such as an automobile, is designed such that a power transmission path in a speed change mechanism adapted to receive as an input an engine output rotation through a torque converter is changed based on selective engagement of a plurality of friction elements to establish a plurality of speed stages different in speed reduction ratio. Recent years, with a view to improvements in driving performance, fuel economy performance and others for vehicles, the number of speed stages has tended to be increased. For example, U.S. Pat. No. 6,176,802 B1 (hereinafter referred to as “Patent Document 1”) and U.S. Patent Publication No. 2006/0135309 A1 (hereinafter referred to as “Patent Document 2”) disclose an automatic transmission intended to establish six forward speed stages.
Specifically, an automatic transmission disclosed in the Patent Document 1 comprises a speed change mechanism which is made up using one single-pinion type planetary gear set and one Ravigneaux type planetary gear set, and an automatic transmission disclosed in the Patent Document 2 comprises a speed change mechanism which is made up using two single-pinion type planetary gear sets and one double-pinion type planetary gear set. Each of these speed change mechanisms is provided with five friction elements, and adapted to establish six forward speed stages and a reverse speed stage based on selective engagement of the five friction elements.
The Ravigneaux type planetary gear set used in the automatic transmission disclosed in the Patent Document 1 has problems about a relatively high cost and a risk of causing gear noise, because of its structural complexity as compared with a single-pinion type planetary gear set, due to the requirement to have two sun gears different in diameter, and two types of pinions.
The double-pinion type planetary gear set used in the automatic transmission disclosed in the Patent Document 2 also has problems about a relatively high cost and an increase in diameter, because of its structural complexity due to the requirement to have two types of pinions. Moreover, in both the above planetary gear sets, the number of gears is inevitably increased to cause an increase in power loss, as compared with a single-pinion type planetary gear set.